Robust mode staggercasting fast channel change

ABSTRACT

A method and apparatus for staggercasting a plurality of content representative signals includes encoding a first and a corresponding second signal representing each of the plurality of content representative signals. A composite signal is generated comprising the plurality of first and second encoded signals. In the composite signal, each respective second encoded signal is delayed with respect to the corresponding first encoded signal. The first and second encoded signal representing a selected one of the content representative signals is extracted to reproduce the selected content representative signal. The extracted first encoded signal is decoded if an error is detected in the extracted second encoded signal, otherwise the extracted second encoded signal is decoded. When a different content representative signal is newly selected, the first extracted encoded signal is decoded until the delayed second extracted encoded signal is available.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/US2004/001769, filed 23 Jan. 2004, whichwas published in accordance with PCT Article 21(2) on Aug. 19, 2004 inEnglish and which claims the benefit of U.S. provisional patentapplication No. 60/443,672, filed Jan. 28, 2003. This application isrelated to copending, commonly assigned, U.S. patent application Ser.Nos. 10/486,400, entitled ROBUST RECEPTION OF DIGITAL BROADCASTTRANSMISSION, filed on Jul. 17, 2002; 11/716,921, entitled ROBUSTRECEPTION OF DIGITAL BROADCAST TRANSMISSION, filed on Mar. 12, 2007;10/543,044 entitled ROBUST MODE STAGGERCASTING, filed on Jan. 20, 2004;10/542,976 entitled ROBUST MODE STAGGERCASTING WITHOUT ARTIFACTS, filedon Jan. 21, 2004; 10/543,043 entitled ROBUST MODE STAGGERCASTING WITHADJUSTABLE DELAY OFFSET, filed on Jan. 21, 2004; 10/543,483 entitledROBUST MODE STAGGERCASTING REDUCED RESOLUTION VIDEO FOR MOBILE RECEIVER,filed on Jan. 22, 2004; 10/543,368 entitled ROBUST MODE STAGGERCASTINGWITH MULTIPLE DELAYS FOR MULTI-RESOLUTION SIGNALS, filed on Jan. 23,2004; 10/542,972 entitled ROBUST MODE STAGGERCASTING USER CONTROLLEDSWITCHING MODES, filed on Jan. 27, 2004; and 10/543,045 entitled ROBUSTMODE STAGGERCASTING STORING CONTENT, filed on Jan. 26, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to staggercasting methods and apparatus.

2. Background of the Invention

Current digital television transmission standards in the United States,as proposed by the Advanced Television Systems Committee (ATSC) datedSep. 16, 1995, incorporated by reference herein, use a single carriermodulation technique: eight level vestigial sideband modulation (8-VSB).Because it is a single carrier modulation technique, it is susceptibleto signal degradation in the communications channel, such as fadingcaused by multipath and other signal attenuation. While some such fadingmay be compensated by channel equalization techniques, if the fade islong enough and severe enough, then the receiver will lose the signaland the demodulator system will lose synchronization. Reacquiring thesignal, and resynchronizing the demodulator can take several seconds andis quite objectionable to a viewer.

To overcome this problem, a first ATSC proposal permits creation of asecond communications channel by permitting use of a more robustmodulation technique for a limited period of time, e.g. less than 10%.For example, a 2 or 4-VSB modulation technique may be used for selectedframes. A second ATSC proposal permits a more robust encoding technique,e.g. trellis encoding, while maintaining an 8-VSB modulation technique.Such a system permits improved performance with compatible receiverswhile maintaining backwards compatibility with existing receivers.

Another known technique for overcoming fading is staggercasting. PCTApplication No. US02/22723 filed Jul. 17, 2002, by K. Ramaswamy, et al.,and PCT Application No. US02/23032 filed Jul. 19, 2002 by J. A. Cooper,et al., incorporated by reference herein, disclose staggercastingcommunications systems. Staggercasting communications systems transmit acomposite signal including two component content representative signals:one of which is delayed with respect to the other. Put another way, oneof the component content representative signals is advanced with respectto the other. The composite signal is broadcast to one or more receiversthrough a communications channel. At a receiver, the advanced-in-timecomponent content representative signal is delayed through a delaybuffer so that it becomes resynchronized in time with the othercomponent content representative signal. Under normal conditions, theundelayed received component content representative signal is used toreproduce the content. If, however, a signal fade occurs, then thepreviously received and advanced-in-time content representative signalin the delay buffer is used to reproduce the content until either thefade ends and the composite signal is available again, or the delaybuffer empties. If the delay period, and the associated delay buffer, islarge enough then most probable fades may be compensated for.

PCT Application No. US02/22723 filed Jul. 17, 2002, by K. Ramaswamy, etal., and PCT Application No. US02/23032 filed Jul. 19, 2002 by J. A.Cooper, et al. also disclose a staggercasting system in which one of thecomponent signals in the composite signal represents the content at ahigher quality than the other component signal. In this arrangement, thelower quality component signal is advanced in time relative to thehigher quality component signal. As described above, at the receiverunder normal conditions, the undelayed received component, which in thiscase is the higher quality component signal, is used to reproduce thecontent. If, however, a signal fade occurs, then the previously receivedand advanced-in-time content representative signal, which in this caseis the lower quality component signal, in the delay buffer is used toreproduce the content until either the fade ends and the compositesignal is available again, or the delay buffer empties. This permitsreproduction of a higher quality signal under normal conditions, andreproduction of a lower quality signal in the presence of a fade event.Because the lower quality signal requires fewer bits to transmit, theoverhead required to provide fade resistance is decreased.

The systems disclosed in PCT Application No. US02/22723 filed Jul. 17,2002, by K. Ramaswamy, et al., and PCT Application No. US02/23032 filedJul. 19, 2002 by J. A. Cooper, et al., may be used to transmit aplurality of content representative signals, each staggercasted using afirst and a delayed second component signal. When a new contentrepresentative signal is selected by a viewer, the delay buffer in thereceiver must fill before the second component signal may be decoded.This delay period is annoying to a viewer and therefore undesirable. Asystem which can decrease the time required between when a new contentrepresentative signal is selected and when it begins to be decoded anddisplayed is desirable.

BRIEF SUMMARY OF THE INVENTION

In accordance with principles of the present invention, a method andapparatus for staggercasting a plurality of content representativesignals includes encoding a first and a corresponding second signalrepresenting each of the plurality of content representative signals. Acomposite signal is generated comprising the plurality of first andsecond encoded signals. In the composite signal, each respective secondencoded signal is delayed with respect to the corresponding firstencoded signal. The first and second encoded signal representing aselected one of the content representative signals is extracted toreproduce the selected content representative signal. The extractedfirst encoded signal is decoded if an error is detected in the extractedsecond encoded signal, otherwise the extracted second encoded signal isdecoded. When a different content representative signal is newlyselected, the first extracted encoded signal is decoded until thedelayed second extracted encoded signal is available.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a portion of a staggercasting transmitter;

FIG. 2 is a block diagram of a portion of a staggercasting receiver;

FIG. 3 is a packet timing diagram useful in understanding the operationof the staggercasting communications system illustrated in FIG. 1 andFIG. 2;

FIG. 4 is a GOP timing diagram useful in understanding the operation ofan enhanced staggercasting communications system;

FIG. 5 is a block diagram of a selector which may be used in thereceiver illustrated in FIG. 2;

FIG. 6 is a block diagram of a portion of another embodiment of astaggercasting receiver;

FIG. 7 is a video frame timing diagram useful in understanding theoperation of the staggercasting receiver illustrated in FIG. 6;

FIG. 8 illustrates an extended syntax and semantics for the program maptable (PMT) and/or program and information systems protocol—virtualchannel table (PSIP-VCT);

FIG. 9 is a block diagram of a portion of another embodiment of astaggercasting transmitter for transmitting multiple resolution versionof a content representative signal;

FIG. 10 is a block diagram of a portion of another embodiment of astaggercasting receiver for receiving a transmitted multiple resolutionversion of a content representative signal;

FIG. 11 is a block diagram of a portion of a transmitter fortransmitting a dual interlaced content representative signal;

FIG. 12 is a block diagram of a portion of a receiver for receiving adual interlaced content representative signal; and

FIG. 13 is a display diagram useful in understanding the operation ofthe dual interlace transmitter illustrated in FIG. 11 and dual interlacereceiver illustrated in FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a portion of a staggercasting transmitter100 according to principles of the present invention. One skilled in theart will understand that other elements, not shown to simplify thefigure, are needed for a complete transmitter. One skilled in the artwill further understand what those elements are and how to select,design, implement and interconnect those other elements with theillustrated elements.

In FIG. 1, a source (not shown) of content, which in the illustratedembodiment may be a video image signal, audio sound image, program data,or any combination of these, provides a content representative signal toan input terminal 105 of the transmitter 100. The input terminal 105 iscoupled to respective input terminals of a robust mode encoder 110 and anormal mode encoder 120. An output terminal of the robust mode encoder110 is coupled to a first input terminal of a multiplexer 140. An outputterminal of the normal mode encoder 120 is coupled to an input terminalof a delay device 130. An output terminal of the delay device 130 iscoupled to a second input terminal of the multiplexer 140. An outputterminal of the multiplexer 140 is coupled to an input terminal of amodulator 150. An output terminal of the modulator 150 is coupled to anoutput terminal 115. The output terminal 115 is coupled to acommunications channel (not shown).

In operation, the normal mode encoder 120 encodes the content video,audio and/or data using a source encoding technique. In the illustratedembodiment, the source encoding technique is the MPEG 2 encodingtechnique, although any such source encoding technique may be used. Thesource encoding process is performed using predetermined parametersincluding resolution, frame rate, quantization level, etc. Furtherprocessing is performed in the normal mode encoder 120 to system encodethe source encoded content representative signal. In the illustratedembodiment, the source coded content representative signal is formedinto a series of transport packets containing the encoded video, audioand/or data. These transport packets are formatted according to the MPEG2 standard, although any such system encoding may be used.

The robust mode encoder 110 also encodes the content video, audio and/ordata, using a source encoding technique. The source encoding techniqueused by the robust mode encoded 110 is more robust compared with thesource encoding technique of the normal mode encoder 120. In theillustrated embodiment, the robust mode encoding used is a video codingtechnique designated MPEG AVC/H.264 currently being developed by theJoint Video Team (JVT) of the ISO/IEC MPEG and ITU-T VCEG committees,and termed JVT coding below. However, any such source encoding techniquemay be used. For example, other source coding techniques, such asenhanced trellis coding, which provide robust encoding relative to theMPEG normal mode encoder 120, may also be used. The robust encodingprocess is also performed using predetermined parameters includingresolution, frame rate, quantization level, etc., but the values ofthese parameters may be different for the robust encoding process thanthose for the normal encoding process. Processing is also performed inthe robust mode encoder 110 to system encode the source encoded contentrepresentative signal. In the illustrated embodiment, the source codedcontent representative signal is formed into a series of transportpackets, also according to the MPEG 2 standard, although, again, anysuch system encoding may be used.

The normal mode encoded signal is delayed by the delay device 130 by anamount intended to allow the system to operate through a range ofexpected fade periods. The value of this parameter depends on thecharacteristics of the communications channel. For example, in an urbansetting, with many buildings and moving objects, such a airplanes,fading is much more common and can last longer than in rural flatsettings. In the illustrated embodiment, the delay may be varied fromaround 0.5 seconds to several seconds.

FIG. 3 is a packet timing diagram useful in understanding the operationof the staggercasting communications system illustrated in FIG. 1 andFIG. 2. FIG. 3 illustrates the system coded transport packet streams atthe input terminal of the multiplexer 140. In FIG. 3, packets from therobust mode encoder 110 are represented by a horizontal row of squares300, labeled using lower case letters: “a”, “b”, “c”, and so forth.Packets from the normal mode encoder 120 are represented by a horizontalrow of squares 310, labeled using numbers: “0”, “1”, . . . , and uppercase letters: “A”, “B”, “C”, and so forth. Packets labeled by the sameletter contain data representing content from the same time. That is,packet “a” from the robust mode encoder 110 contains data representingcontent which corresponds in time to the content represented by the datain packet “A” from the normal mode encoder 120. Each packet in thenormal mode and robust mode packet streams contains data in the headeridentifying them as belong to that packet stream. The delay device 130delays the normal mode encoder 120 packets by a time delay T_(adv). Thatis, robust mode packets are advanced in time by T_(adv) with respect tocorresponding normal mode packets. In the embodiment illustrated in FIG.3, T_(adv) is ten packet time periods. This time period may vary fromaround 0.5 seconds to several seconds, as described above.

The robust mode and delayed normal mode packet streams are multiplexedtogether into a composite packet stream in the multiplexer 140. Thecomposite packet stream is time domain multiplexed, meaning that asingle data stream carrying successive packets, one at a time, isproduced. Additional packets containing other data, such asidentification and control data (not shown), may also be multiplexedinto the composite packet stream produced by the multiplexer 140. Inaddition, other packet streams representing other content sources (alsonot shown), possibly including both normal mode and robust mode packetstreams representing one or more of the other content representativesignals, may also be multiplexed into the composite packet streamproduced by the multiplexer 140, all in a known manner. The packetstreams 300 and 310 in FIG. 3 represent the component contentrepresentative signals in the composite packet stream. As may be seen,packet “A” from the normal mode encoder 120 is transmitted at the sametime as packet “k” from the robust mode encoder 110.

The composite packet stream from the multiplexer 140 is then channelcoded for transmission over the communications channel. In theillustrated embodiment, the channel coding is done by modulating thecomposite packet stream in the modulator 150. The channel coding for thenormal mode packet stream is different from the channel coding for therobust mode packet stream. More specifically, the modulation applied tothe robust mode packet stream is more robust than that applied to thenormal mode packet stream. In the illustrated embodiment, when packetsin the normal mode packet stream are modulated, the modulation is 8-VSBmodulation according to the ATSC standard. When packets in the robustmode packet stream are modulated, the modulation is more robustmodulation, for example 4-VSB or 2-VSB, as described above.

In short, in the illustrated embodiment, the normal mode packet streamis source encoded using the MPEG 2 encoding technique and is channelencoded using 8-VSB modulation. This is fully backward compatible withthe prior ATSC standard. Also in the illustrated embodiment, the robustmode packet stream is source encoded using the JVT encoding techniqueand is channel encoded using 4-VSB and/or 2-VSB modulation. One skilledin the art will understand that the new ATSC standard, referred toabove, refers only to the channel encoding of the robust mode packetstream, i.e. 4-VSB and/or 2-VSB, and does not specify a source encodingtechnique. Consequently, any such source encoding technique may be usedaccording to the standard, and the JVT encoding technique in theillustrated embodiment is one example of such source encoding for therobust mode packet stream. In the remainder of this application, ‘normalmode packet stream’ will refer to the packet stream which is sourceencoded using the MPEG 2 source encoding technique, system encoded intopackets according to the MPEG 2 standard, and channel encoded using8-VSB modulation; and ‘robust mode packet stream’ will refer to packetswhich are source encoded using the JVT source encoding technique, systemencoded into packets according to the MPEG 2 standard, and channelencoded using 4-VSB and/or 2-VSB modulation.

The modulated composite signal is then supplied to the communicationschannel (not shown), which may be a wireless RF channel, or a wiredchannel, such as a cable television system. The composite signal may bedegraded by the communications channel. For example, the signal strengthof the composite signal may vary. In particular, the composite may fadedue to multipath or other signal attenuation mechanisms. One or morereceivers receive the possibly degraded composite signal from thecommunications channel.

FIG. 2 is a block diagram of a portion of a staggercasting receiver 200according to principles of the present invention. In FIG. 2, an inputterminal 205 is connectable to the communications channel (not shown) sothat it is capable of receiving the modulated composite signal producedby the transmitter 100 (of FIG. 1). The input terminal 205 is coupled toan input terminal of a demodulator 207. An output terminal of thedemodulator 207 is coupled to an input terminal of a demultiplexer 210.A first output terminal of the demultiplexer 210 is coupled to aselector 230. A second output terminal of the demultiplexer 210 iscoupled to a delay device 220. An output terminal of the delay device220 is coupled to a second input terminal of the selector 230. An outputterminal of the selector 230 is coupled to a signal input terminal of amulti-standard decoder 240. A control signal output terminal of thedemultiplexer 210 is coupled to respective corresponding input terminalsof the selector 230 and the multi-standard decoder 240. An outputterminal of the multi-standard decoder 240 is coupled to an outputterminal 215 The output terminal 215 produces a content representativesignal which is supplied to utilization circuitry (not shown) such as atelevision receiver with an image reproduction device to reproduce theimage represented by the video content, a sound reproduction device toreproduce the sound represented by the audio content, and possiblyincluding user input devices to allow a viewer to interact with thereceived data content.

In operation, the demodulator 207 demodulates the received modulatedsignal using the appropriate demodulation techniques required to receivepackets from either the normal mode packet stream (8-VSB) or robust modepacket stream (4-VSB and/or 2-VSB). The resulting signal is a receivedcomposite packet stream signal. The received composite packet streamsignal is demultiplexed by the demultiplexer 210 into respective normalmode source encoded and robust mode source encoded component packetstreams according to the identification data in the header of eachreceived packet. The received normal mode packet stream is supplieddirectly to the selector 230. The received robust mode packet stream ispassed through the delay device 220, which delays the received robustmode packet stream by the same time duration that, in the transmitter100 of FIG. 1, the normal packet stream is delayed. Consequently, thecontent represented by the two packet stream signals at the inputterminals of the selector 230 is time aligned.

The demultiplexer 210 also produces an error signal at the controlsignal output terminal should a portion of the received composite signalbe unusable. Any of several techniques may be used, for example, asignal-to-noise ratio detector or a bit-error rate detector. Inaddition, an error in the received composite signal may be detected bydetecting missing packets. Each packet includes in its header both dataidentifying which packet stream the packet belongs to and a packetsequence number. If a sequence number for a packet stream is missed,then a packet is missing, and an error is detected. In this case, thepacket stream from which the packet is missing may be noted, and onlythat packet stream detected as having an error. These or any other suchdetector may be used, alone or in combination.

Although the control signal is illustrated as emanating from thedemultiplexer 210, one skilled in that art will understand thatdifferent error detectors may be require signals from different placesin the receiver. Whatever arrangement is used, an error signal E isgenerated which is active when a portion of the composite signal isunusable. The selector 230 is conditioned to pass one of the two packetstreams signals to the multi-standard decoder 240 in response to thiserror signal E. The multi-standard decoder 240 is conditioned to decodethat packet stream signal, in a manner to be described in more detailbelow.

The multi-standard decoder 240 performs both system decoding(depacketizing) and source decoding of whichever packet stream issupplied to it by the selector 230. The multi-standard decoder 240 canbe configured to perform source decoding of the packet stream signalsaccording to different coding standards. For example, when a normal modeencoded packet stream is received from the selector 230, themulti-standard decoder 240 is configured to depacketize and sourcedecode these packets according to the MPEG 2 standard and regenerate thecontent representative signal. Similarly, when a robust mode encodedpacket stream is received from the selector 230, the multi-standarddecoder 240 is configured to depacketize the packets according to theMPEG 2 standard and to source decode these packets according to the JVTstandard, and regenerate the content representative signal.

The operation of the receiver 200 of FIG. 2 may be understood byreferring again to FIG. 3. Time t0 may represent the time when thereceiver is turned on, or when a user specifies a new content source toreceive. During the time, T_(adv), between t0 and t4, robust modepackets “a” to “j” are loaded into the delay device 220, and normal modepackets, designated “0” though “9” are received. At time t4, the normalmode packet “A” becomes available from the demultiplexer 210 and delayedrobust mode packet “a” becomes available from the delay device 220.Under normal conditions, the error signal is not active on the errorsignal line E. In response, the selector 230 couples the normal modepacket stream to the multi-standard decoder 240, and the multi-standarddecoder 240 begins to generate the content representative signal fromthe normal mode packets, as described above. This is illustrated by thecross hatching 301 in the normal mode packets “A” through “G”.

From time t1 to t2 a severe fade occurs in the communications channeland from time t2 to t3 the receiver recovers the modulated signal andresynchronizes to that signal. During this time, from t1 to t3, normalmode packets “H” to “M” and robust mode packets “r” to “w” are lost.This is indicated by the cross hatching 302 and 303 in those packets.However, robust mode packets “h” to “m” have been previouslysuccessfully received. Because of the delay device 220, these robustmode packets are available at the other input to the selector 230 fromtime t1 to t3.

The occurrence of the fade is detected and indicated by an active errorsignal on the error signal line E. In response to the active errorsignal on the error signal line E, the selector 230 couples thepreviously received robust mode packets “h” to “m” to the multi-standarddecoder 240. Concurrently, the multi-standard decoder 240 is configuredto depacketize and decode robust mode packets. Consequently, from timet1 to t3, packets “h” to “m” from the robust mode packet stream aredecoded and the content representative signal remains available to theutilization circuitry (not shown). This is illustrated by the crosshatching 301 in the robust mode packets “h” through “m”.

At time t3, the fade ends and the composite signal becomes availableagain. Consequently the normal mode packets “N”, “O”, “P”, . . . ,become available. The disappearance of the fade is detected andindicated by an inactive error signal on the error signal line E. Inresponse, the selector 230 couples the normal mode packet stream to themulti-standard decoder 240. Concurrently, the multi-standard decoder 240is configured to depacketize and decode normal mode packets andcontinues to generate the content representative signal.

During the fade and recovery, from time t1 to t3, robust packets “r”through “w” were lost. Consequently, from time t6 through t7, whennormal mode packets “R” through “W” are received, there are nocorresponding robust mode packets in the delay device 220. During thistime, there is no protection against a fade. However, once the delaydevice is refilled, fade protection becomes available again.

As described above, the content representative signal remains availableto the utilization circuitry (not shown) despite the occurrence of afade from time t1 to t3. In addition, because of the robust sourcecoding and channel coding (modulation) techniques, the robust modepackets are likely to survive more severe channel degradation, and thusbe available when normal mode packets may not be. The quality of thecontent signal carried by the robust mode packet stream may be differentfrom that in the normal mode packet stream. In particular, the qualityof the content signal in the robust mode packet stream may be lower thanthat in the normal mode packet stream. A lower quality content signalrequires fewer bits to transmit than a higher quality content signal,and such a robust mode packet stream will require a lower throughputthan the normal mode packet stream. Thus, at the expense of a second,lower throughput packet stream, a system which will permit a gracefuldegradation in the event of a fading event is possible.

Also as described above, the content signal may include video, audioand/or data. In particular, audio data may be carried in both the normalmode packet stream and the robust mode packet stream so that audio dataalso remains available despite the occurrence of a fade. The audiocontent signal carried by the robust mode packet stream may have adifferent quality, specifically a lower quality, than that in the normalmode packet stream. An audio signal at a lower quality may be carried byfewer bits and fewer packets, and, thus, would make relatively lowrequirements on the robust mode packet stream. This also would permit agraceful degradation in the event of a fade event.

With a system described above, switching from the normal mode packetstream to the robust mode packet stream may occur at any time. If therobust packet stream carries content representative data which isidentical to that in the normal packet stream down to the packet level,this may not present a problem. However, if the robust packet streamcarries content representative data which is different from that in thenormal packet stream, for example, if the content is represented at adifferent resolution, quantization level, frame rate, etc., then theviewer may notice a change in the reproduced image which may beobjectionable. In a worse case, if a packet stream switch occurs in themiddle of decoding a picture, then the decoding of that picture andother surrounding pictures may fail altogether, and the video image maybe disrupted for a much longer period of time, until the decoderresynchronizes to an independently decodable picture.

As described above, the normal mode packet stream is carried by acombination of source, system and channel encoding. In the illustratedembodiment, the source and system coding is according to the known MPEG2 coding scheme and the channel encoding uses the 8-VSB modulationtechnique. The MPEG source coding scheme encodes a video image signal asa sequence of independent decoding segments. An independent decodingsegment (IDS), also termed an elementary stream segment, is a segmentwhich may be decoded accurately independent of any other independentdecoding segment. In the MPEG standard, independent decoding segmentsinclude a sequence, group of pictures (GOP) and/or picture. Theseindependent decoding segments are delimited in the compressed bitstreamby unique start codes. That is, an independent decoding segment isconsidered to be all the data beginning with a segment start code, up tobut not including the next segment start code. Pictures in the MPEG 2standard are either intra-coded (I pictures), inter-prediction (Ppictures) or bi-directional prediction (B) pictures. I pictures areencoded without reference to any other pictures. A GOP includes a groupof pictures encoded as a combination of I, P, and/or B pictures. In aclosed GOP, all pictures in the GOP may be decoded without reference topictures in any other GOP. The start of each GOP is clearly identifiedin the MPEG 2 packet stream.

Also as described above, the robust mode packet stream is carried by acombination of source, system and channel encoding. In the illustratedembodiment, the source encoding is according to the JVT encoding scheme,the system encoding is according to the MPEG 2 standard and the channelencoding uses the 2-VSB and/or 4-VSB modulation techniques. Picturescoded using the JVT source coding standard are made up of coded slices,and a given picture may contain slices of different coding types. Eachslice may be an intra-coded (I) slice, an inter-predictive (P) slice, abi-predictive (B) slice, an Si slice in which only spatial prediction isused, or an SP slice which may be accurately reproduced even whendifferent reference pictures are used. The JVT source coding standardalso includes an instantaneous decoding refresh (IDR) picture. An IDR isa particular type of JVT encoded picture, which contains only I slicesand marks the beginning of an IDS. An IDR indicates that the currentpicture, and all later encoded pictures may be decoded without requiringreference to previous pictures. An IDR may be encoded once for everypredetermined number of pictures, emulating a GOP in the MPEG 2standard. In the JVT source encoding scheme, independent decodingsegments may be delimited by IDRs, which are clearly identified in theJVT packet stream.

By imposing some constraints on the normal and robust source encodingschemes, a system may be developed which can switch from the normal modepacket stream to the robust mode packet stream while minimizingobjectionable artifacts. If independent decoding segments are encoded tobegin at identical content locations in both the normal (MPEG 2) androbust (JVT) packet streams, switches may be made between the normal androbust packet streams at independent decoding segment locations withminimal objectionable artifacts. In the illustrated embodiment, theindependent decoding segment used in the normal (MPEG 2) packet streamis a closed GOP and begins with an I picture. In the correspondingrobust (JVT) packet stream, each independent decoding segment beginswith an IDR picture. The I picture in the normal (MPEG) mode packetstream and the IDR picture in the robust (JVT) mode packet stream bothencode the same video picture from the content source (not shown). Bothsource encoding schemes permit IDSs to be formed and delimited in othermanners. For example, the MPEG 2 source encoding scheme also permitsslices to be formed to represent a picture. Any such manner may be usedprovided that IDSs are inserted into both packet streams at identicalcontent locations.

Referring again to FIG. 1, the input terminal 105 is further coupled toan input terminal of a scene cut detector 160, illustrated in phantom.An output terminal of the scene cut detector 160 is coupled torespective control input terminals of the normal mode encoder 120 andthe robust mode encoder 110.

In operation, the scene cut detector 160 detects the occurrence of a newscene in the video content. In response to detection of a new scene, acontrol signal is sent to the normal mode encoder 120 and the robustmode encoder 110. Both the normal mode encoder 120 and the robust modeencoder 110 begin encoding a new independent decoding segment inresponse to the control signal. The normal mode encoder 120 inserts anew I picture and the robust mode encoder 110 inserts an IDR pictureinto their respective encoded packet streams. The normal mode encoder120 and the robust mode encoder 110 operate to generate correspondingindependent decoding segments having the same time durations. Asdescribed above, the encoded content representative signals are systemcoded into respective packet streams.

The delay device 130 is set to introduce a delay equal to theindependent decoding segment time duration. The multiplexer 140 combinesthe robust mode encoded packet stream and the delayed normal modeencoded packet stream into a composite packet stream. The compositepacket stream is channel encoded (modulated) in an appropriate manner bythe modulator 150 and supplied to the communications channel via theoutput terminal 115.

The operation of the transmitter in this mode of operation may be betterunderstood by reference to FIG. 4. FIG. 4 illustrates the packet streamsat the input to the multiplexer 140. In FIG. 4, a sequence ofindependent decoding segments (IDS) from the robust mode encoder 110 isillustrated as a series of rectangles 400, and a sequence of independentdecoding segments from the normal mode encoder 120 is illustrated as aseries of rectangles 410. As described above, the time locations withinthe content, and the durations of the independent decoding segments fromthe robust mode encoder 110 and the normal mode encoder 120 are thesame. Because the delay introduced by the delay device 130 is the sameas the time duration of an IDS, IDSs from the robust mode encoder 110align with the immediately preceding IDS from the normal mode encoder120.

At time t0, which may represent a change in scene, as detected by thescene cut detector 160, the undelayed robust mode encoded IDS N beginsand the previously delayed normal mode encoded IDS N−1 begins. Eachrobust mode (JVT source coded) IDS is illustrated as a series ofrectangles 440 representing respective slices, and begins with anindependent decoding refresh (IDR) picture. The IDR picture is followedby B, P, SI, and/or SP slices. These slices are, in turn, system codedinto a sequence 450 of transport packets “a”, “b”, “c”, etc. Similarly,each normal mode IDS (MPEG 2 source coded) is illustrated as a series ofrectangles 420 representing a GOP which begins with an I picture. The Ipicture is followed by an arrangement of P pictures and B pictures.These I, P and B pictures are, in turn, system coded into a sequence 430of transport packets “A”, “B”, “C”, etc. The illustrated arrangementsare examples only, and any appropriate arrangement may be used.

This composite signal is received by a receiver. Referring again to thereceiver 200 in FIG. 2, at time t0, the received robust mode IDS N isloaded into the delay device 220 during time T_(adv). The delay device230 introduces the same delay (one IDS time period) to the receivedrobust packet stream that in the transmitter the delay device 130introduced into the normal packet stream. Consequently, the receivednormal packet stream and delayed robust packet stream at the inputterminals of the selector 230 are realigned in time with respect to thecontent representative signal.

Under normal conditions, the selector 230 couples the normal mode packetstream to the multi-standard decoder 240, and the multi-standard decoderis conditioned to decode normal mode packets, as described in moredetail above. If an error is detected in the composite signal or aportion of it, as described above, then switching is performed betweenthe normal mode packet stream and the robust mode packet stream. In thisembodiment, at the beginning of the IDS, the selector 230 couples therobust mode packet stream to the multi-standard decoder 240, and themulti-standard decoder 240 is conditioned to decode robust mode packets,as described in more detail above. If no further errors are detected inthe composite signal, then at the beginning of the next IDS, theselector 230 couples the normal mode packet stream to the multi-standarddecoder 240 and the multi-standard decoder 240 is conditioned to decodenormal mode packets again.

In the receiver 200 in FIG. 2 switching from decoding the normal modepacket stream to decoding the robust mode packet stream and vice versaoccurs at the beginning of an IDS. Each IDS is an independent decodingsegment, beginning with either an I picture (normal mode) or an IDRpicture (robust mode), which may be successfully decoded withoutreference to any other picture. Further, subsequent pictures may bedecoded without reference to pictures preceding the IDS. Thus, decodingand display of the content representative signal may be immediatelyperformed without objectionable artifacts caused by the switching.

To further minimize video artifacts caused by switching from decoding anormal mode video packet stream to a robust mode packet stream, and viceversa, the image characteristics of the resulting video signal may begradually changed between those of the normal mode video signal andthose of the robust mode video signal when a switch occurs. This isespecially desirable when the robust mode video stream is lower qualitycompared to the normal mode video stream, for example, if the spatialresolution, frame rate, etc. of the robust mode video stream is lessthan that of the normal mode video stream.

FIG. 5 is a block diagram of a selector 230″ which may be used in thereceiver illustrated in FIG. 3. Such a selector 230″ may graduallychange the video characteristics (e.g. resolution, frame rate, etc.) ofthe resulting video signal between those of the normal mode video signaland those of the robust mode video signal at the time of a switchbetween them. FIG. 5 a is a functional diagram which illustrates theoperation of selector 230″, and FIG. 5 b is a structural block diagramillustrating an embodiment of such a selector 230″ which may be used inthe receiver illustrated in FIG. 2.

In FIG. 5 a, the robust mode video signal is coupled to one end of atrack 232 and the normal mode video signal is coupled to the other endof the track 232. A slider 234 slides along the track 232 and generatesa resulting video signal which is coupled to the output terminal of theselector 230″. The resulting video signal is coupled to the outputterminal 215 of the receiver 200 (of FIG. 2). A control input terminalis coupled to receive the error signal E from the demultiplexer 210. Thecontrol input terminal is coupled to an input terminal of a controllercircuit 231. The position of the slider 234 along the track 232 iscontrolled by the controller circuit 231, as indicated in phantom.

In operation, when the slider 234 is at the upper end of the track 232,then a resulting video signal having the characteristics (e.g.resolution, frame rate, etc.) of the robust mode video signal is coupledto the output terminal of the selector 230″. When the slider 234 is atthe lower end of the track 232, then a resulting video signal having thecharacteristics of the normal mode video signal is coupled to the outputterminal of the selector 230″. As the slider 234 moves between the upperend and the lower end of the track 232, then the characteristics of theresulting video signal at the output terminal of the selector 230″ isadjusted to be between those of the normal mode and robust mode videosignals. The closer the slider 234 is to the upper end of the track 232,the closer the characteristics of the resulting video signal are thoseof the robust mode video signal than to those of the normal mode videosignal. The closer the slider 234 is to the lower end of the track 232,the closer the characteristics of the resulting video signal are thoseof the normal mode video signal than to those of the robust mode videosignal.

The value of the error signal E indicates when a switch is to occur, asdescribed above. When a switch occurs from one video signal (e.g. thenormal mode or robust mode video signal) to the other video signal, fora time interval of one or more video pictures around the time when theswitch occurs, the slider 234 is gradually moved from one end of thetrack 232 to the other. For example, during a switch from the normalmode video signal to the robust mode video signal, the slider 234 beginsat the bottom of the track. For several video pictures before theswitch, the slider gradually moves from the bottom of the track 232 tothe top. At the time of the switch from the normal mode packet stream tothe robust mode packet stream, the slider is at the top of the track232. Consequently, the characteristics of the resulting video signalgradually change from those of the normal video signal to those of therobust mode video signal during several video pictures before the switchto the robust mode packet stream occurs. Similarly, at the time of theswitch from the robust mode packet stream to the normal mode packetstream, the slider is at the top of the track 232. For several videopictures after the switch, the slider gradually moves from the top ofthe track 232 to the bottom. Consequently, the characteristics of theresulting video signal gradually change from those of the robust videosignal to those of the normal mode video signal during several videopictures after the switch to the normal mode packet stream occurs.

In FIG. 5 b, the video signal from the multi-standard decoder 240 (ofFIG. 2) is coupled to a first input terminal of a variable video qualityfilter 236 and a first input terminal of a selector 238. An outputterminal of the video quality filter 236 is coupled to a second inputterminal of the selector 238. An output terminal of the selector 238generates the resulting video signal and is coupled to the outputterminal 215 (of FIG. 2). The error signal E from the demultiplexer 210is coupled to a controller circuit 231. A first output terminal of thecontroller circuit 231 is coupled to a control input terminal of thevideo quality filter 236 and a second output terminal of the controllercircuit 231 is coupled to a control input terminal of the selector 238.

In operation, the video characteristics of the decoded video signal isvaried by the video quality filter 236 in response to the control signalfrom the controller circuit 231. The control signal from the controllercircuit 231 conditions the video quality filter 236 to produce a videosignal having a range of video characteristics between those of thenormal mode video signal and those of the robust mode video signal.Under normal conditions, when no switching occurs, the controllercircuit 231 conditions the selector 238 to couple the decoder videosignal to the output terminal as the resulting video signal.

In response to a change in the value of the error signal E, indicating aswitch between the normal mode video signal and the robust mode videosignal as described above, for a time interval near the switch time thecontroller circuit 231 conditions the selector 238 to couple the videosignal from the video quality filter 236 to the output terminal andconditions the quality filter 236 to gradually change the videocharacteristics of the resulting video signal. More specifically, if aswitch from the normal mode video signal to the robust mode video signaloccurs, for a time interval of several video pictures before the switchoccurs the video quality filter 236 is conditioned to gradually changethe video characteristics of the resulting video signal from those ofthe normal video signal to those of the robust video signal. At thebeginning of that time interval, the selector 238 is conditioned tocouple the filtered video signal to the output terminal as the resultingvideo signal. When that time interval is complete, and the decoded videosignal is derived from the robust mode packet stream, the selector 238is conditioned to couple the decoded video signal to the output terminalas the resulting video signal. Similarly, if a switch from the robustmode video signal to the normal mode video signal occurs, for a timeinterval of several video pictures after the switch occurs the videoquality filter 236 is conditioned to gradually change the videocharacteristics of the resulting video signal from those of the robustvideo signal to those of the normal video signal. At the beginning ofthat time interval, the selector 238 is conditioned to couple thefiltered video signal to the output terminal as the resulting videosignal. When that time interval is complete, and the decoded videosignal is derived from the normal mode packet stream, the selector 238is conditioned to couple the decoded video signal to the output terminalas the resulting video signal.

Abrupt switching between video signals having different video quality(resolution, frame rate, etc.) may cause artifacts which may beobjectionable to a viewer. Because the video quality of the resultingvideo signal is gradually reduced before switching from the normal modevideo signal to the robust mode video signal and gradually increasedafter switching from the robust mode video signal to the normal modevideo signal, objectionable artifacts resulting from the switch areminimized.

Another embodiment of a staggercasting communications system may alsoprovide switching while minimizing objectionable artifacts and does notrequire any special placement of IDSs in the normal and robust modepacket streams. A receiver 200′ is illustrated in FIG. 6. In FIG. 6,elements which are similar to those in the receiver 200 in FIG. 2 aredesignated by the same reference number and are not described in detailbelow. In FIG. 6, the first output terminal of the demultiplexer 210 iscoupled to the input terminal of the normal mode decoder 240′. A firstoutput terminal of the normal mode decoder 240′ is coupled to the firstinput terminal of the selector 230′ and a second output terminal of thenormal mode decoder 240′ is coupled to a first input terminal of anormal mode frame store 250′. The output terminal of the delay device220 is coupled to the input terminal of the robust mode decoder 240″. Afirst output terminal of the robust mode decoder 240″ is coupled to thesecond input terminal of the selector 230′ and a second output terminalof the robust mode decoder 240″ is coupled to a first input terminal ofa robust mode frame store 250″. The output terminal of the selector 230′is coupled to respective second input terminals of the normal mode framestore 250′ and the robust mode frame store 250″. An output terminal ofthe normal mode frame store 250′ is coupled to a second input terminalof the normal mode decoder 240′ and an output terminal of the robustmode frame store 250″ is coupled to a second input terminal of therobust mode decoder 240″.

In operation, the delay device 220 introduces the same delay into therobust mode packet stream that the delay device 130 in the transmitter100 (of FIG. 1) introduces into the normal mode packet stream.Consequently, the packet stream signals at the respective inputterminals of the normal mode decoder 240′ and the robust mode decoder240″ are time aligned with respect to the content representative signal.

Both the normal and the delayed robust mode packet streams are systemand source decoded to produce corresponding content representativesignal streams, as described in detail above. In the illustratedembodiment, these content representative signal streams are respectivesequences of video pictures. In both normal mode decoding and robustmode decoding, video data representing surrounding pictures are requiredto decode predictive pictures or slices. The normal mode frame store250′ holds these surrounding pictures for the normal mode decoder 240′and the robust mode frame store 250″ holds these surrounding picturesfor the robust mode decoder 250″.

In the receiver illustrated in FIG. 6, switching is performed on apicture-by-picture basis rather than on an IDS basis. The normal modedecoder 240′ decodes normal mode packets into an associated contentrepresentative signal containing successive video pictures.Concurrently, the robust mode decoder 240″ decodes robust mode packetsinto an associated content representative signal containing successivevideo pictures. As described above, the demultiplexer 210 produces anerror signal on the error signal line E indicating that the compositesignal from the demodulator 207, or at least some portion of it, isunusable. In the embodiment illustrated in FIG. 6, this error signal maybe generated by detecting missing packets in the demultiplexed packetstreams. Thus, the error signal on the error signal line E indicates notonly that a packet is missing but also which packet stream is missing apacket. Because the packets carry in the payload a portion of the dataforming a video picture carried by the packet stream, and carry data inthe header identifying the packet stream to which this packet belongs,the packet stream which is missing a packet may be marked as erroneous.

A video picture may be successfully received in both the normal androbust mode packet streams; may be successfully received in the normalmode packet stream but erroneously received in the robust mode packetstream; may be erroneously received in the normal packet stream butsuccessfully received in the robust packet stream; or may be erroneouslyreceived in both the normal and robust mode packet streams.

Under normal conditions, that is, when no error is detected in eitherthe normal mode nor the robust mode packet streams, both the normal modedecoder 240′ and the robust mode decoder 240″ successfully decode thecorresponding video picture. The selector 230′ couples the contentrepresentative video picture derived from the normal mode decoder 240′to the output terminal 215. Also, under normal conditions, the normalmode decoder 240′ supplies video pictures to the normal mode frame store250′ and the robust mode encoder 240″ supplies video pictures to therobust mode frame store 250″.

If an error is detected in the robust mode packet stream but no error isdetected in the normal mode packet stream, then only the normal modedecoder 240′ successfully decodes the corresponding video picture. Theselector 230′ couples the content representative video picture derivedfrom the normal mode decoder 240′ to the output terminal 215. Also, thenormal mode decoder 240′ supplies the decoded video picture to thenormal mode frame store 250′. However, because the robust mode decoder240″ did not successfully decode the corresponding video picture, itdoesn't supply any video picture to the robust mode frame store 250″.Instead, the successfully decoded video picture from the normal modedecoder 240′ is routed from the selector 230′ to the robust mode framestore 250″.

If an error is detected in the normal mode packet stream but no error isdetected in the robust mode packet stream, then only the robust modedecoder 240″ successfully decodes the corresponding video picture. Theselector 230′ couples the content representative video picture derivedfrom the robust mode decoder 240″ to the output terminal 215. Also, therobust mode decoder 240″ supplies the decoded video picture to therobust mode frame store 250″. However, because the normal mode decoder240′ did not successfully decode the corresponding video picture, itdoesn't supply any video picture to the normal mode frame store 250′.Instead, the successfully decoded video picture from the robust modedecoder 240″ is routed from the selector 230′ to the robust mode framestore 250′.

In the above two cases, the video picture stored in the frame storeassociated with the decoder which did not successfully decode that videopicture is the video picture from the other decoder. This may degradesubsequent decoding compared to what it would be if the correct videopicture were stored in the frame store. This is especially true shouldthe substituted video picture be of lower quality than the erroneousvideo picture. However, the accuracy of subsequent decoding is betterthan if no video picture at all were stored in the frame store.

Should an error be detected in a video picture in both the normal modeand robust mode packet stream then no accurate video picture is decodedand other masking techniques must be performed.

The operation of the receiver 200′ illustrated in FIG. 6 may be betterunderstood by reference to FIG. 7. In FIG. 7, a top set of rectangles(MPEG) respectively represent the input 420 and output 520 of the normalmode decoder 240′; a middle set of rectangles (JVT) respectivelyrepresent the input 440 and output 540 of the robust mode decoder 240″;and the bottom set of rectangles (OUTPUT) respectively represent thevideo pictures 460 and their source 560 at the output terminal 215.Referring to the MPEG decoding: the upper set of rectangles 420represent the source coded video pictures (I, P, and/or B) at the inputterminal of the normal mode decoder 240′. The lower set of rectangles520 represent the resulting video pictures at the output terminal of thenormal mode decoder 240′. Similarly, referring to the JVT decoding: theupper set of rectangles 440 represent the source coded IDR picture(which may include a plurality of only I slices) and the followingsource coded video slices (I, P, B, SI and/or SP) at the input terminalof the robust mode decoder 240″. The lower set of rectangles 540represent the resulting video pictures at the output terminal of therobust mode decoder 240″. Referring to the output terminal 215, theupper set of rectangles 460 represent the output video pictures and thelower set of rectangles 560 represent the source of that particularvideo picture.

More specifically, in the normal mode (MPEG) packet stream, the videopictures 6, 10 and 13 are each missing at least one packet, as indicatedby crosshatching. Similarly, in the robust mode (JVT) packet stream, thevideo pictures 7 and 10 are missing at least one packet, as indicated bythe crosshatching. All the other video pictures for both the normal modeand robust mode packet streams include all packets and may besuccessfully decoded.

For video pictures 0-5, 8, 9, 11, 12 and 14, the selector 230′ couplesthe video pictures derived from the normal mode decoder 240′ (MPEG) tothe output terminal 215, as indicated by “M” in FIG. 7. In addition, forthese video pictures, the video pictures from the normal mode decoder240′ are supplied to the normal mode frame store 250′ and the videopictures from the robust mode decoder 240″ are supplied to the robustmode frame store 250″.

For pictures 6 and 13, the video pictures in the normal mode packetstream are erroneous but the corresponding video pictures in the robustmode packet stream are complete and available. For these pictures, theselector 230′ couples the video picture from the robust mode decoder240″ (JVT) to the output terminal 215, as indicated by “J” in FIG. 7.Because for these pictures there is no normal mode video picture, therobust mode video picture from the robust mode decoder 240″ is coupledto both the robust mode frame store 250″ and the normal mode frame store250′.

For picture 7, the video picture in the normal mode packet stream iscomplete but the corresponding video picture in the robust mode packetstream is erroneous. For this picture, the selector 230′ couples thevideo picture from the normal mode decoder 240′ to the output terminal215, as indicated by “M” in FIG. 7. Because for this picture there is norobust mode video picture, the normal mode video picture from the normalmode decoder 240′ is coupled to both the normal mode frame store 250′and the robust mode frame store 250″.

For picture 10, the video picture in both the normal mode and robustmode packet streams is erroneous. Because there is no valid videopicture, some form of error masking may be used. This is indicated by an“XX” in FIG. 7. Because there is no valid video picture from either thenormal mode decoder 240′ or the robust mode decoder 240″, no decodedvideo picture may be stored in either the normal mode frame store 250′or the robust mode frame store 250″. The data stored in the frame stores250′ and 250″ may also be derived from some form of error masking.

By decoding both packet streams into streams of video pictures, andswitching from one video stream to the other at the beginning of eachvideo picture, video artifacts resulting from failure to properly decodea packet stream may be minimized. Switching with a gradual change ofvideo quality, as illustrated in FIG. 5 may be used in a receiver asillustrated in FIG. 6. However, because in the receiver of FIG. 6switching occurs at each picture, artifacts from such switching are notas objectionable as when switching occurs at IDS boundaries, as in FIG.2.

Degraded channel conditions may, however, result in frequent switchesbetween normal mode and robust mode packet streams. This frequentswitching may result in artifacts which may be objectionable to aviewer. This is especially true if the video quality of the robust modevideo signal is substantially different from that of the normal modevideo signal.

In order to minimize artifacts caused by over-frequent switching betweenthe normal mode packet stream and the robust mode packet stream, theselector 230 (of FIG. 2) and 230′ (of FIG. 6) is configured to restrictswitching at more often than a predetermined frequency. Morespecifically, the selector 230 or 230′ may monitor the frequency atwhich switching is desired, and compare it to a predetermined threshold.If the frequency of desired switching is over the threshold, then thefrequency at which actual switching occurs is restricted to below somemaximum frequency. This is a form of switching hysteresis.

For example, assume that the normal mode packet stream carries a videosignal of high quality (e.g. high definition (HD)) and the robust modepacket stream carries a video signal of lower quality (e.g. standarddefinition (SD)). When the normal mode HD packet stream is unavailable,then the robust mode SD packet stream is processed to generate theimage. Upscaling an SD video signal for display on an HD display devicegenerates a video image of poor quality. If the normal mode packetstream is fading in and out frequently, but the robust mode packetstream remains available, then frequent switches between the normal modeHD video signal and the robust mode SD video signal occur. Frequentswitches between HD and SD packet streams, with frequent switchesbetween high quality and low quality images, produce artifacts which areobjectionable to a viewer.

Continuing the example, if the error signal E indicates that switchingshould occur (i.e. normal mode packets are missing) e.g. more than twotimes per minute, then actual switching is restricted to minimize theswitching artifacts described above. In this example, under theseconditions the selector 230 or 230′ selects the robust mode packetstream for e.g. at least one minute for every switch. This will decreasethe number of switches and, thus, minimize the visible artifactsresulting from those switches. One skilled in the art will understandthat this is only one embodiment implementing switching hysteresis. Thethresholds for the maximum switching frequency to invoke hysteresis andfor the restricted switching frequency may be made different than thoseof the example. Such thresholds may be determined empirically to findthose which minimize objectionable visible artifacts. Further, thethresholds may be dynamically varied during the operation of thereceiver. Finally, other hysteresis algorithms may be developed torestrict switching in the presence of conditions which would normallyresult in excessive switching.

Referring again to FIG. 3 and FIG. 4, at the beginning of any broadcastor channel change, there is a period designated T_(adv) during which thenormal mode packets (310, 410) are filling the delay device 220 (of FIG.2 and FIG. 6). In the receivers illustrated in FIG. 2 and FIG. 6, onlyafter the delay circuit 220 is full does the receiver begin operation.However, this causes undue delay when a receiver is switched on or achannel is changed. During the time interval T_(adv), however, therobust mode packet stream (300, 400) is immediately available.

In FIG. 2, the undelayed robust mode packet stream is coupled directlyfrom the demultiplexer 210 to a third input terminal of the selector230, as illustrated in phantom. When the receiver is powered on or a newchannel is selected, the selector 230 couples the undelayed robust modepacket stream to the multi-standard decoder 240. The multi-standarddecoder 240 is conditioned to depacketize and decode the robust modepackets, as described in detail above, and a video signal is madeimmediately available to the utilization circuitry at output terminal215. When the normal mode packet stream becomes available, then theselector 230 will couple the normal mode packet stream signal to themulti-standard decoder 240.

In FIG. 6, the undelayed robust mode packet stream is coupled directlyfrom the demultiplexer 210 to the robust mode decoder 240″. When thereceiver is powered on or a new channel is selected, the robust modedecoder 240″ is conditioned to depacketize and decode the robust modepacket stream from the demultiplexer 210 and generate a robust modevideo signal, as described in more detail above. The selector 230′ isconditioned to couple the robust mode video signal from the robust modedecoder 240″ to the utilization circuitry via the output terminal 215.When the normal mode packet stream becomes available, then the normalmode decode 240′ depacketizes and decodes it and produces a normal modevideo signal. The selector 230′ is conditioned to couple the normal modevideo signal to the utilization circuitry via the output terminal 215.

In either case, data in the normal mode and robust mode packet streamsare analyzed to determine when the normal mode packet stream has becomeavailable and normal operation of the receiver may be commenced. Inaccordance with known MPEG 2 system (transport packet) encoding,information related to the system time clock (STC) in the transmitter isplaced in the encoded packet streams via program clock reference (PCR)data. Further information, termed a presentation time stamp (PTS), whichindicates when a portion (termed an access unit) of a packet stream mustbe decoded, is included at least at the beginning of each such accessunit. When the normal mode and robust mode packet streams aredepacketized and decoded by the multi-standard decoder 240 (FIG. 2) orthe normal mode decoder 240′ and the robust mode decoder 240″ (FIG. 6),the system time clock (STC) in the receiver is synchronized to that inthe transmitter through the PCR data. When the value of the PTS in thenormal mode packet stream is equal to the value of the receiver STC,this indicates that the normal mode packet stream is in synchronism withthe robust mode packet stream, and the receiver may begin normaloperation by decoding the normal mode packet stream, as described above.

Because many content representative signals may be transmitted on onemultiplexed transport packet stream, a known means for supplyinginformation about the different packet streams has been developed. Eachpacket stream is identified by a packet identifier (PID), which isincluded in the header of each packet in that packet stream. One packetstream, having a predetermined known PID, contains one or more datatables containing identification and other information about all theother packet streams. This known table structure may be used to carryinformation about robust mode packet streams which are not related toany other normal mode packet stream. However, additional informationmust be sent from the transmitter to the receivers about robust packetstreams which are related to other normal mode packet streams.

An extended syntax and semantics for these existing tables may carry thenecessary data. FIG. 8 is a table which illustrates an extended syntaxand semantics for the program map table (PMT) and/or program andinformation systems protocol—virtual channel table (PSIP-VCT). Each rowin FIG. 8 represents either a data item in the extended table, or ameta-syntactical description in pseudo-code form. The first column iseither a name of a data item or a meta-syntactical specification. Thesecond column is a description of the data item or syntacticalspecification. The third column is an indication of the size of any dataitem.

The first item 802 in the extended syntax is the number of robust packetstreams used to staggercast other normal mode packet streams. Theninformation for each such staggercast robust mode packet stream isincluded in the table, as indicated by the meta-syntactic specificationin the next row and the last row of the table. Some such information isrequired for every robust mode packet stream. For example, data 804represents the program identifier (PID) for the robust mode packetstream; data 806 represents the type of data being carried by thatpacket stream; data 808 represents the PID of the normal mode packetstream associated with this packet stream; and data 810 represents thedelay being introduced into the normal mode packet stream by the delaydevice 130 in the transmitter 100 (of FIG. 1).

Some such information, however, relates to robust mode packet streamsonly of a particular data type. For example, if the robust mode packetstream carries video data, then information 812 related to thecompression format, frame rate, interlace format, horizontal andvertical resolution, and bit rate is sent from the transmitter to thereceivers so that the video image represented by the robust mode packetstream may be properly decoded and displayed. Similarly, if the robustmode packet stream carries audio data, the information 814 related tothe compression format, bit rate, sample rate; and audio mode (surround,stereo, or mono) is sent from the transmitter to the receivers so thatthe sound represented by the robust mode packet stream may be properlydecoded and reproduced.

One other piece of data relates to the relative quality of the contentrepresentative signal carried by the robust mode packet stream. Asdescribed above, the quality of the content representative signalcarried by the robust mode packet stream may be different from that ofthe normal mode packet stream with which it is associated. In theexamples described above, the quality of content representative signalcarried by the robust mode packet is specified to be lower than that ofthe associated normal mode packet stream. However, under someconditions, the provider may transmit a higher quality signal on therobust mode packet stream. In this condition, it is preferred thatreceivers use the content representative signal carried by the robustmode packet stream instead of the associated normal mode packet stream.This is indicated to the receivers by the data 816.

By providing information associating robust mode packet streams tonormal mode packet streams, a receiver 200 (of FIG. 2) or 200′ (of FIG.6) may find both the normal mode and robust mode packet streams in themultiplexed packet stream, and concurrently process both of them asdescribed above. Prior receivers which do not include the capabilitiesof the receivers of FIG. 2 and FIG. 6 will ignore this information andprocess the normal mode packet stream in the known manner.

As described above, the delay introduced between the robust mode packetstream and the associated normal mode packet stream by the delay device130 in the transmitter 100 (of FIG. 1) is transmitted as the data 810 inthe table illustrated in FIG. 8. This permits the transmitter to changethe delay period and permits the receiver to adjust its delay periodaccordingly. For example, under some channel conditions fading may bemore likely than others, or the characteristics of the fading may change(i.e. the fades may be longer). Under such conditions, the delay periodmay be increased. The length of the delay is transmitted to thereceivers, which will adapt the delay devices 220 (in FIG. 2 and FIG. 6)to the same delay period. Other conditions may also require differingdelay periods.

The staggercasting concept described above may be expanded. Multipleversions of the same content representative signal, encoded into videosignals having different video quality (e.g. resolution, frame rate,etc.), may be staggercasted. FIG. 9 is a block diagram of a portion ofanother embodiment of a staggercasting transmitter for transmittingmultiple versions of a content representative signal. In FIG. 9 thoseelements which are the same as those in the transmitter illustrated inFIG. 1 are designated by the same reference number and are not describedin detail below. FIG. 10 is a block diagram of a portion of acorresponding embodiment of a staggercasting receiver. In FIG. 10, thoseelements which are the same as those in the receiver illustrated in FIG.2 are designated by the same reference number and are not described indetail below.

In FIG. 9 a, input terminal 105 is coupled to an input terminal of ahierarchical encoder 160. Hierarchical encoder 160 source encodes andpacketizes a plurality of output packet stream signals. A first one (0)of the plurality of output packet stream signals is coupled to acorresponding input terminal of the multiplexer 140′. The remainder ofthe plurality of output packet stream signals, (1) to (n) are coupled torespective input terminals of a corresponding plurality of delay devices130(1) to 130(n). The delay period introduced by the delay device 130(2)is greater than that introduced by delay device 130(1); the delay periodintroduced by the delay device 130(3) (not shown) is greater than thatintroduced by delay device 130(2); and so forth. The delays may bespecified in terms of packets, as illustrated in FIG. 3; independentdecoding segments, as illustrated in FIG. 4; or video picture periods,as illustrated in FIG. 7. Respective output terminals of the pluralityof delay devices 130(1) to 130(n) are coupled to corresponding inputterminals of the multiplexer 140′.

In operation, the first packet stream signal (0) carries a base videosignal source encoded at a lowest video quality. The second packetstream signal (1) carries extra video information. This extra videoinformation, when combined with the base video signal (0) produces avideo signal with a higher video quality than that of the base videosignal (0) alone. The third packet stream signal (2) carries furtherextra video information. The video information in this signal, whencombined with the base video signal (0) and the video information in thesecond packet stream signal (1) produces a video signal with a highervideo quality than that of the combination of the base signal (0) andthe second signal (1). The video information in the additional packetstream signals, up to packet stream signal (n) from the hierarchicalencoder 160, may be combined to produce video signals of higher videoquality. The multiplexed signal is channel encoded (modulated) andsupplied to receivers via output terminal 115.

FIG. 10 a is the receiver corresponding to the transmitter illustratedin FIG. 9 a. The demultiplexer 210 extracts a plurality (0) to (n) ofpacket streams. Packet stream (n) is coupled to a corresponding inputterminal of a hierarchical decoder 260. The remainder (0) to (n−1) (notshown) of the plurality of packet streams are coupled to respectiveinput terminals of a corresponding plurality 220 of delay devices. Theplurality 220 of delay devices are conditioned to realign all of theplurality (0) to (n) of packet streams in time at the input terminals ofthe hierarchical decoder 260. The error signal on signal line E from thedemultiplexer 210 is coupled to a control input terminal of thehierarchical decoder 260. An output terminal of the hierarchical decoder260 is coupled to the output terminal 215.

In operation, the demodulator 207 channel decodes (demodulates) thereceived signal as appropriate, as described in more detail above. Themultiplexer 210 extracts the plurality, (0) to (n), of packet streamscarrying the hierarchy of video information corresponding to the packetstreams (0) to (n) illustrated in FIG. 9 a. These packet streams arealigned in time by the plurality 220 of delay devices. The error signalfrom the demultiplexer 210 indicates which packet streams areunavailable, e.g. missing packets. The plurality of packet streams aredepacketized and the highest quality video image which may behierarchically decoded from the available packet streams is produced bythe hierarchical decoder 260. That is, if a fading event has made allbut the packet stream (0) carrying the base video signal unavailable,then the hierarchical decoder 260 depacketizes and decodes only thepacket stream (0). If the packet stream (1) is also available, then thehierarchical decoder 260 depacketizes and decodes both the packet stream(0) and the packet stream (1) and generates a video signal of higherquality, and so forth. If all packet streams (0) to (n) are available,then the hierarchical decoder 260 depacketizes and decodes them all andgenerates a video signal of the highest video quality.

In FIG. 9 b, the input terminal 105 is coupled to respective inputterminals of a plurality 170 of video encoders. The output terminal of afirst one 170(0) of the plurality 170 of video encoders is coupled to acorresponding input terminal of the multiplexer 140′. The outputterminals of the remainder, 170(1) to 170(n), of the plurality 170 ofvideo encoders are coupled to respective input terminals of a pluralityof delay devices 130(1) to 130(n). The delay period introduced by thedelay device 130(2) is greater than that introduced by delay device130(1); the delay period introduced by the delay device 130(3) (notshown) is greater than that introduced by delay device 130(2); and soforth. The delays may be specified in terms of packets, as illustratedin FIG. 3; independent decoder segments, as illustrated in FIG. 4; orvideo frame periods, as illustrated in FIG. 7. Respective outputterminals of the plurality of delay devices are coupled to correspondinginput terminals of the multiplexer 140′.

In operation, the first encoder 170(0) source encodes the contentrepresentative signal and system encodes (packetizes) the resultingsource encoded signal to generate a packet stream carrying informationrepresenting a video signal at lowest quality: in the illustratedembodiment, a quarter-common-interface-format (QCIF) video signal. Thesecond encoder 170(1) similarly generates a packet stream carryinginformation representing a video signal at a higher quality than thatproduced by the first encoder 170(0): in the illustrated embodiment, acommon-interface-format (CIF) video signal. Other video encoders, notshown, similarly generate packet streams carrying video signals atsuccessively higher video quality. An SD video encoder 170(n−1)similarly generates a packet stream carrying an SD quality video signaland an HD video encoder 170(n) similarly generates a packet streamcarrying an HD quality video signal. These packet streams aremultiplexed by the multiplexer 140′ then channel encoded (modulated) andtransmitted to the receivers via the output terminal 115.

FIG. 10 b is the receiver corresponding to the transmitter illustratedin FIG. 9 b. In FIG. 10 b, the demultiplexer 210 extracts a plurality(0) to (n) of packet streams. The packet stream (n) is coupled to aninput terminal of a HD decoder 270(n). The remainder of the packetstreams (0) to (n−1) are coupled to respective input terminals of aplurality 220 of delay devices. Respective output terminals of theplurality 220 of delay devices are coupled to corresponding inputterminals of a plurality 270 of video decoders. Respective outputterminals of the plurality 270 of video decoders are coupled tocorresponding input terminals of a selector. The error signal on theerror signal line E from the demultiplexer 210 is coupled to a controlinput terminal of the selector 280.

In operation, the demodulator 207 channel decodes (demodulates) thereceived composite signal as appropriate, as described in more detailabove. The demultiplexer 210 extracts the packet streams (0) to (n)corresponding to those generated by the plurality 170 of video encodersillustrated in FIG. 9 b. The plurality 220 of delay devices realigns allthese packet streams (0) to (n) in time at the respective inputterminals of the plurality 270 of video decoders. Each packet stream iscoupled to the video decoder appropriate for decoding the video signalcarried by that packet stream. For example, the packet stream carryingthe QCIF quality video signal is coupled to the QCIF decoder 270(0); thepacket stream carrying the CIF quality video signal is coupled to theCIF decoder 270(1) and so forth. Each video decoder in the plurality 270of video decoders depacketizes and source decodes the signal supplied toit to generate a video signal. The error signal E from the demultiplexer210 indicates which of the packet streams (0) to (n) is unavailable dueto errors (e.g. missing packets). The selector 280 is conditioned tocouple the highest quality video signal produced from available packetstreams to the output terminal 215.

One skilled in the art will understand that image scaling may berequired for some of the lower quality video image signals in thetransmitter systems illustrated in FIG. 9. The encoders, either thehierarchical encoder 160 of FIG. 9 a or the plurality 170 of encoders ofFIG. 9 b, include any such image scaling circuitry which is necessary itis not shown to simply the figure.

For the communications system illustrated in FIG. 9 and FIG. 10, any ofthe packet streams produced by the hierarchical encoder 160 (of FIG. 9a) or any of the plurality 170 of video encoders (of FIG. 9) may besource encoded according to the robust source encoding scheme (JVT) andchannel encoded (modulated) by the robust modulation scheme (4-VSBand/or 2-VSB), as described in more detail above. The correspondingdemodulation and decoding of that packet stream takes place in thereceiver of FIG. 10. Also, the lowest quality video signal is advancedthe most, and consequently has the highest fade resistance. Further, thelowest video quality signal may be encoded with the least number of bitsand thus takes a small amount of time to transmit. As the video qualityof the video signal carried by packet streams increases, the time bywhich that packet stream is advanced decreases, consequently the faderesistance decreases. Thus, when the channel characteristic has nofades, then the packet stream(s) carrying the highest video qualitysignal remain(s) available. Mild fades leave packet stream(s) carryinglower video quality signals available, and severe fades leave only thepacket stream carrying the lowest quality video signal available. Thisgradual reduction in video quality as channel characteristics degrade isa desirable characteristic for a viewer.

As described above, and illustrated in FIG. 1 and FIG. 9 b, the samecontent representative signal may be staggercasted as a packet streamcarrying a high quality video signal and as one or more packet streamscarrying reduced video quality video signals. In such a communicationssystem, it is, therefore, possible for some receivers, for example, atelevision receiver in a cellular phone or personal digital assistant(PDA), to extract and decode only a reduced quality contentrepresentative signal. In such a receiver, the display device is lowerresolution and may only be able to display a reduced quality videosignal. Further, the use of battery power makes it advantageous tominimize the amount of data processed. Both of these considerationssuggest that such receivers decode only the packet stream carrying avideo signal of appropriate video quality and display that image.

FIG. 10 c illustrates a receiver. In FIG. 10 c, the input terminal 205is coupled to the input terminal of the demodulator 207. An outputterminal of the demodulator 207 is coupled to the input terminal of thedemultiplexer 210. An output terminal of the demultiplexer 210 iscoupled to an input terminal of a decoder 270. An output terminal of thedecoder is coupled to the output terminal 215.

In operation, the demodulator 207 demodulates the received compositesignal in the appropriate manner, as described in more detail above. Thedemultiplexer 210 selects only a single packet stream having a videosignal of the desired quality. For example, this may be a QCIF formatvideo signal, such as produced by the QCIF encoder 170(0) of FIG. 9 band carried on packet stream (0). The packet stream (0) is extracted bythe demultiplexer 210 and is decoded by the decoder 270 to produce theQCIF format video signal. Such a receiver need only receive the tableillustrated in FIG. 8 to determine the PID of the desired lower qualityvideo signal packet stream (0). From the resolution data 812 transmittedin the table, the mobile receiver is able to select the packet streamcarrying the reduced quality video signal desired for processing.

The communications system illustrated in FIG. 9 and FIG. 10 may befurther extended. In the systems described above, video informationcarried in additional packet streams, may be used to provide gracefuldegradation under worsening channel conditions. However, such systemsmay also transmit additional video information which can enhance thequality of video signals under good channel conditions. By including apacket stream carrying augmented video information, in addition to thepacket stream carrying the normal video signal, an augmented video imagemay be transmitted.

FIG. 11 is a block diagram of a portion of a transmitter fortransmitting a dual interlaced video signal and FIG. 12 is a blockdiagram of a portion of a receiver for receiving a dual interlaced videosignal. FIG. 13 is a display diagram useful in understanding theoperation of the dual interlace transmitter illustrated in FIG. 11 andthe dual interlace receiver illustrated in FIG. 12. In FIG. 11, thoseelements which are the same as those illustrated in FIG. 1 aredesignated by the same reference number and are not described in detailbelow. In FIG. 12, those elements which are the same as thoseillustrated in FIG. 6 are designated by the same reference number andare not described in detail below.

Referring to FIG. 13, a content source produces a progressive scan videodisplay, illustrated schematically at the top of FIG. 13 as a sequenceof video lines 1310 within a display border 1320. A normal HD videoimage picture includes 1080 lines. Such an HD video image is transmittedat a rate of 30 frames per second in interlaced format. That is, aninterlacer generates two fields: a first field including only oddnumbered lines and a second field including only even numbered lines.These fields are transmitted successively at a rate of 60 fields persecond.

In FIG. 11, the input terminal 105 is coupled to a dual outputinterlacer 102. A first output terminal of the dual output interlacer102 is coupled to the input terminal of the robust mode encoder 110. Asecond output terminal of the dual output interlacer 102 is coupled tothe input terminal of the normal mode encoder 120.

Referring again to FIG. 13, the frame display image 1330(A) correspondsto the video signal A produced at the first output terminal of the dualoutput interlacer 102 and the frame display image 1330(B) corresponds tothe video signal B produced at the second output terminal of the dualoutput interlacer 102. In the frame display images 1330(A) and 1330(B),solid lines are transmitted in one field, and dotted lines aretransmitted in the following field. In the frame display image in1330(A) solid lines are odd lines and dotted lines are even lines; andin the frame display image 1330(B), solid lines are even lines anddotted lines are odd lines. This is illustrated in more detail in thefield display images 1340(A), 1340(B), 1350(A) and 1350(B) beneath theframe display images 1330 (A) and 1330(B). In field 1, video signal Atransmits the odd lines as illustrated in field display image 1340(A),and video signal B transmits the even lines, as illustrated in fielddisplay image 1340(B). In field 2, the video signal A transmits the evenlines as illustrated in field display image 1350(B) and the video signalB transmits the odd lines as illustrated in field display image 1350(B).

As described in more detail above, the video signal A is source encodedusing JVT source encoding, then system encoded (packetized) by therobust mode encoder 110. The video signal B is source encoded using MPEG2 source encoding, then system encoded (packetized) by the normal modeencoder. The modulator channel encodes (modulates) the robust modepacket stream using 4-VSB and/or 2-VSB modulation, and modulates thenormal mode packet stream using 8-VSB modulation.

In FIG. 12, a first output terminal of the demultiplexer 210 is coupledto the input terminal of the normal mode decoder 240′ and a secondoutput terminal of the demultiplexer 210 is coupled to the inputterminal of the delay device 220. The output terminal of the normal modedecoder 240′ is coupled to a first signal input terminal of a dual inputdeinterlacer 202 and the output terminal of the robust mode decoder 240″is coupled to a second signal input terminal of the dual inputdeinterlacer 202. The error signal from the demultiplexer 210 is coupledto a control input terminal of the dual input deinterlacer 202. Anoutput terminal of the dual input deinterlacer 202 is coupled to theoutput terminal 215.

As described in more detail above, the demodulator 207 channel decodes(demodulates) the robust mode packet stream using 4-VSB and/or 2-VSBdemodulation and demodulates the normal mode packet stream using 8-VSBdemodulation. The normal mode decoder 240′ system decodes (depacketizes)and source decodes the normal mode packet stream using JVT decoding toreproduce the video signal B. The robust mode decoder 240″ depacketizesand source decodes the robust mode packet stream using MPEG 2 decodingto reproduce the video signal A.

The dual input deinterlacer 202 operates to combine the interlaced scanlines of the video signal A from the robust mode decoder 240″ with theinterlaced scan lines of the video signal B from the normal mode decoder240′ to produce a progressive scan field. For field 1, the odd scanlines from video signal A, illustrated in field display image 1340(A),are combined with the even scan lines from video signal B, illustratedin field display image 1340(B). The resulting progressive scan field isillustrated in the field display image 1345. For field 2, the even scanlines from video signal A, illustrated in field display image 1350(A),are combined with the odd scan lines from video signal B, illustrated infield display image 1350(B). The resulting progressive scan field isillustrated in the field display image 1355. Thus, a progressive scanfield may be produced at the output terminal of the dual inputdeinterlacer 202 each field period. For an HD signal, this means that afull 1080 line image is produced 60 times per second.

The dual interlaced technique described above and illustrated in FIG.11, FIG. 12 and FIG. 13 may also be combined with the techniquesdescribed above to provide a wider range of graceful degradation in theevent channel conditions degrade. If channel conditions render one ofthe packet streams carrying video signals A or B unavailable, then theerror signal E indicates this to the dual input deinterlacer 202. Thedual input deinterlacer 202 begins producing the standard HD interlacedvideo signal from the available video signal. The display device (notshown), is reconfigured to display the image represented by the standardinterlaced video signal until the other video signal becomes availableagain. If neither of the HD video signals are available, then thehighest quality available video signal may be displayed, as described indetail above with reference to the transmitter in FIG. 9 and thereceiver in FIG. 10.

The same technique may also be used to convert any interlaced formatvideo signal, for example an SD video signal, to a progressive scanvideo signal at twice the frame rate. It is not necessary for the twovideo signals A and B to be staggercasted, as illustrated in FIG. 11 andFIG. 12. It is only necessary that they be simulcasted. However,staggercasting additionally provides graceful degradation in thepresence of fade events, as described above.

The communications system described above may be further extended tocooperate with a recording device, such as a digital personal videorecorder (PVR). Such PVR devices are becoming included in digitaltelevision receivers due to the decreasing costs of such a device. InFIG. 9 b, a PVR device 295 includes a video terminal (Vid)bidirectionally coupled to the selector 280, and a control terminal(Ctl) also bidirectionally coupled to the selector 280, as illustratedin phantom. The selector 280 is also coupled to a source of usercontrol, also as illustrated in phantom.

The selector 280 is configured to couple any desired video signal fromthe plurality 270 of video detectors to the PVR 295 independently of theinput video signal coupled to the output terminal 215. The selector 280may also be configured to couple an input video signal from the PVR 295to the output terminal 215 for playback. The selector 280 may alsosupply control data to the PVR 295, and the PVR 295 supply status datato the selector 280 over the bidirectional control terminal.

The PVR 295 may be controlled in several modes of operation. In one modeof operation, the best available video signal is coupled to the PVR 295for recording. In this operational mode, the selector 280 couples thesame input video signal to the PVR 295 as is coupled to the outputterminal 215. This will result in the best quality video signal beingrecorded, but will take the most storage space, in the PVR 295. Thiswill take advantage of the normal mode and robust mode packet streamscarrying the video signal and the graceful degradation that provides.Alternatively, a lower resolution video signal may be coupled to the PVR295 than is coupled to the output terminal 215. For example, while theselector 280 may couple the best available video signal to the outputterminal 215, the selector 280 may couple a video decoder 270 producinga lesser quality video signal to the PVR 295. This lesser quality videosignal may be a selected one of the available video signals, such as theSD quality video signal from the SD decoder 270(n−1), with gracefuldegradation supplied by the lesser quality video decoders. Such a signalwill require less storage space in the PVR 295 than the best availablevideo signal. This will help to conserve storage space in the PVR 295,and allow for longer recording times. In the event that the selectedlower quality video signal becomes unavailable, a higher quality signalmay be recorded until the lower quality signal becomes available again.The selection of which lesser quality video to record (i.e. SD, or CIFor QCIF) may be directly selected by a viewer via the user inputterminal. Alternatively, the selector 280 may automatically control thisselection according to some criterion. For example, a status signal fromthe PVR 295 can indicate the amount of storage remaining in the PVR 295.As the amount of storage remaining drops, the selector 280 mayautomatically couple a video decoder 270 having reduced video quality tothe PVR 295. Other criteria may be derived and used to control whichvideo signal is coupled to the PVR 295 by the selector 280.

Similarly, a user may desire to control the selection and display of thetelevision programs being broadcast by a transmitter. In existingbroadcasting systems, one of the transmitted packet streams carries auser program guide, containing information about all programs currentlybeing broadcast and those due to be broadcast in the near future. Fromthe program guide data, an image of a table listing all such programs,their channels and times may be generated by an on-screen displaygenerator (OSD) 282 as illustrated in FIG. 10 b. A user may control thedisplay of the program guide information as an aid in finding a desiredprogram and selecting that program to view using a user interface. Theuser interface displays images to present information to a viewer,requests input from a viewer and accepts viewer input from controlswhich may be incorporated in the receiver or in a remote control.Existing systems allow a viewer to request additional information abouta program listing, such as a more detailed description of the program, arating (G, PG, R, etc.), time duration, time remaining and so forth.

Additional information related to the staggercasting system describedabove may be added to the displayed program table, or theadditional-information display. This information may be derived from thePSIP-VCT/PMT tables illustrated in FIG. 8. For example, additionalindicators may be added to the displayed program table and/oradditional-information display indicating that: this program is beingstaggercasted; what the video quality is of the video signals beingstaggercasted; what the audio quality of the audio signals beingstaggercasted; and so forth. By displaying this information for aviewer, the viewer is able to base selection of a program on it. Morespecifically, a viewer may select a program that is being staggercasted;or may select a program having video signal of a desired video quality,e.g. to match the display device to which the signal is being supplied.

Current receivers also allow a viewer to set certain parameters. Forexample, a user may wish to automatically view all transmitted channels,or only channels to which the viewer is subscribed, or the subscribedchannels plus pay-per-view channels, and so forth without having tomanually change the on-screen-display each time it is displayed. A userinterface presents a user with a screen image, via the OSD 282, on whichthis selection may be made using the user controls. An additional screenimage may be produced, or an existing screen image modified, on which aviewer sets choices about selection and display of video signals whichhave been staggercasted, as described above. For example, a viewer mayselect to have the program table display only staggercasted programs, orto display staggercasted programs carrying video signals at or above aminimum video quality.

1. A method for use in a receiving apparatus for staggercasting aplurality of content representative signals, comprising using thereceiving apparatus to perform the steps of: for each of the pluralityof content representative signals: encoding a first signal representingthe content; and encoding a second signal corresponding to the firstencoded signal; generating a composite signal comprising the pluralityof first and second encoded signals wherein each respective secondencoded signal is delayed with respect to the corresponding firstencoded signal; extracting the first and second encoded signalrepresenting a selected one of the content representative signals toreproduce the selected content representative signal; decoding theextracted first encoded signal if an error is detected in the extractedsecond encoded signal, otherwise decoding the extracted second encodedsignal; and when a different content representative signal is newlyselected, decoding the first extracted encoded signal until the delayedsecond extracted encoded signal is available; wherein for each of theplurality of content representative signals, the steps of encoding thefirst and second encoded signals comprises inserting time informationinto both the first and second encoded signals; and when a differentcontent representative signal is newly selected, the step of decodingthe first extracted encoded signal until the delayed second extractedencoded signal is available comprises the steps of: extracting the timeinformation from the extracted first and second encoded signals; anddetermining when the delayed second extracted encoded signal isavailable using the extracted time information.
 2. A method for use in areceiving apparatus for staggercasting a plurality of contentrepresentative signals, comprising using the receiving apparatus toperform the steps of: for each of the plurality of contentrepresentative signals: encoding a first signal representing thecontent; and encoding a second signal corresponding to the first encodedsignal; generating a composite signal comprising the plurality of firstand second encoded signals wherein each respective second encoded signalis delayed with respect to the corresponding first encoded signal;extracting the first and second encoded signal representing a selectedone of the content representative signals to reproduce the selectedcontent representative signal; decoding the extracted first encodedsignal if an error is detected in the extracted second encoded signal,otherwise decoding the extracted second encoded signal; and when adifferent content representative signal is newly selected, decoding thefirst extracted encoded signal until the delayed second extractedencoded signal is available; and wherein the step of encoding the secondencoded content representative signal comprises the step of usingencoding robust relative to the encoding of the first encoded contentrepresentative signal; and wherein the first and second encoded contentrepresentative signals are channel encoded and the channel encoding ofthe second content representative signal is robust relative to thechannel encoding of the first content representative signal; and whereinthe step of encoding the first content representative signal comprisesusing 8-VSB modulation; and the step of encoding the second contentrepresentative signal comprises using one of 4-VSB or 2-VSB modulation;and the step of decoding the extracted first and second encoded signalscomprises channel decoding the first encoded signal using 8-VSBdemodulation and channel decoding the second encoded signal using one of4-VSB or 2-VSB demodulation.
 3. A staggercasting receiver, for receivinga composite signal comprising a plurality of first and correspondingsecond encoded signals representing a corresponding plurality of contentrepresentative signals, wherein each respective second encoded signal isdelayed with respect to the corresponding first encoded signal,comprising: a demultiplexer, responsive to the composite signal, forextracting the first and second encoded signals representing a selectedone of the plurality of content representative signal and generating anerror signal representing an error in the first and second encodedsignals; a decoder, coupled to the demodulator, for: decoding theextracted first encoded signal if an error is detected in the extractedsecond encoded signal, otherwise decoding the extracted second encodedsignal; and when a different content representative signal is newlyselected, decoding the first extracted encoded signal until the delayedsecond extracted encoded signal is available; and wherein each of theplurality of first and second encoded signals includes time information;and the decoder extracts the time information from the extracted firstand second encoded signals, and determines that the delayed secondextracted encoded signal is available using the extracted timeinformation; and wherein the time information in the first and secondencoded signal is a presentation time stamp (PTS); the receiver furthercomprises a program clock reference (PCR), coupled to the decoder; andwhen a different content representative signal is newly selected, thePCR is synchronized to the extracted PTS from the first extractedencoded signals; and when a different content representative signal isnewly selected, the delayed second extracted encoded signal is deemedavailable when the PTS from the second extracted encoded signal is equalto the PCR.
 4. A staggercasting receiver, for receiving a compositesignal comprising a plurality of first and corresponding second encodedsignals representing a corresponding plurality of content representativesignals, wherein each respective second encoded signal is delayed withrespect to the corresponding first encoded signal, comprising: ademultiplexer, responsive to the composite signal, for extracting thefirst and second encoded signals representing a selected one of theplurality of content representative signal and generating an errorsignal representing an error in the first and second encoded signals; adecoder, coupled to the demodulator, for: decoding the extracted firstencoded signal if an error is detected in the extracted second encodedsignal, otherwise decoding the extracted second encoded signal; and whena different content representative signal is newly selected, decodingthe first extracted encoded signal until the delayed second extractedencoded signal is available; and wherein one of the first and secondencoded signals is encoded using a coding technique which is robustrelative to the coding technique used for the other one of the first andsecond encoded signals; and the decoder comprises a normal mode decoderfor decoding the first encoded signal and a robust mode decoder fordecoding the robust second encoded signal; and wherein the first andsecond encoded signals are channel encoded and the channel codingtechnique used for the second encoded signal is robust relative to thechannel coding technique used for the first encoded signal; and thenormal mode decoder comprises a channel decoder, for channel decodingthe first encoded signal; and the robust mode decoder comprises achannel decoder, for channel decoding the second encoded signal; andwherein the first encoded signal is channel encoded using 8-VSBmodulation and the second encoded signal is channel encoded using one of4-VSB or 2-VSB modulation; and the first decoder comprises an 8-VSBdemodulator; and the second decoder comprises a demodulator for one of4-VSB or 2-VSB demodulation.
 5. A method for use in a receivingapparatus for processing a received staggercasted signal represented bya composite signal comprising a plurality of first and correspondingsecond encoded signals representing a corresponding plurality of contentrepresentative signals, wherein each respective second encoded signal isdelayed with respect to the corresponding first encoded signal,comprising using the receiving apparatus to perform the steps of:extracting the first and second encoded signals representing a selectedone of the plurality of content representative signal and generating anerror signal representing an error in the first and second encodedsignals; decoding the extracted first encoded signal if an error isdetected in the extracted second encoded signal, otherwise decoding theextracted second encoded signal; and when a different contentrepresentative signal is newly selected, decoding the first extractedencoded signal until the delayed second extracted encoded signal isavailable; and wherein each of the plurality of first and second encodedsignals includes time information; and the extracting step extracts thetime information from the extracted first and second encoded signals,and determines that the delayed second extracted encoded signal isavailable using the extracted time information; and wherein the timeinformation in the first and second encoded signal is a presentationtime stamp (PTS); a program clock reference (PCR) is synchronized to theextracted PTS from one of the first and second extracted encodedsignals; and when a different content representative signal is newlyselected, the PCR is synchronized to the extracted PTS from the firstextracted encoded signal and the delayed second extracted encoded signalis deemed available when the PTS from the second extracted encodedsignal is equal to the PCR.